This recipe can easily be halved: Boil the potatoes in a large saucepan and bake them in an 8-inch square baking dish. If made ahead, the casserole will be slightly more firm and set than a freshly made one; we liked both versions. If you'd like to serve 5 to 6 people, see How to Cut This Recipe in Half below.

FOR THE POTATOES: Adjust oven rack to upper-middle position and heat oven to 400 degrees. Place potatoes and 2 tablespoons salt in Dutch oven, add water to cover by 1 inch, and bring to boil over high heat. Reduce heat to medium and simmer until potatoes are tender and can be easily pierced with paring knife, 18 to 20 minutes.

Drain potatoes and return them to pot. Add butter and mash with potato masher until smooth and no lumps remain. Stir in half-and-half, pepper, and salt until fully combined. Stir in mozzarella, blue cheese, bacon, and rosemary until incorporated. Transfer potato mixture to 13 by 9-inch baking dish and smooth top with spatula. (To make ahead, let potato mixture cool completely in dish, cover tightly with plastic wrap, and refrigerate for up to 3 days. When ready to bake, remove plastic, cover dish tightly with aluminum foil, and bake for 25 minutes. Remove foil and proceed with step 3.)

FOR THE TOPPING: Combine bread crumbs and melted butter in bowl. Sprinkle topping evenly over potato mixture.

Bake until casserole is heated through and topping is crisp and golden brown, about 30 minutes. Let cool for 30 minutes. Serve.

Why Yukon Golds?

How to Cut This Recipe In Half